1. Field of the Invention
The present invention relates to an optical waveguide device and an optical modulator used for an optical communication, and in particular, to an optical waveguide device and an optical modulator, each of which is provided with a function as a polarizer.
2. Description of the Related Art
At the present day, there are realized variety types of optical transmission systems each using an optical waveguide device provided with various functions in optical communications. For example, in an optical transmission system of high-speed and long-distance, an optical modulator is widely used, in which an optical waveguide, an electrode and the like are formed on a lithium niobate (LiNbO3; LN) substrate being electro-optic crystal or the like.
FIG. 29 shows one example of a structure of typical LN optical modulator. In this LN optical modulator, an electrode is arranged on two arms of a Mach-Zehnder interferometer (MZI) type waveguide, and a phase difference between two waveguides is controlled according to a voltage applied on the electrode, so that the ON/OFF of an output signal can be realized. It is typical that a buffer layer is disposed between the waveguides and the electrode in order to suppress the optical absorption by the electrode. Generally, a drive voltage (Vπ) for such an optical modulator is inverse proportion to the electrode length.
In the LN optical modulator as described in the above, since r33 is the largest constant among electro-optic constants (tensor) of the LiNbO3 crystal, only a light of which electric field is polarized to a z-axis direction of the crystal (a TE mode in a x-cut substrate, and a TM mode in a z-cut substrate) is utilized. Therefore, it is typical to input the light into the crystal by adjusting a polarized wave of an incident light to the z-axis direction using a polarization-maintaining fiber, for example.
However, it is impossible to avoid that the polarized wave (the TM mode in the x-cut substrate, and the TM mode in the z-cut substrate) orthogonal to the z-axis direction is incident or excited, due to the axis deviation between a modulator chip and an input fiber, the polarization variation of the incident light, the lack of polarization-extinction ratio in the waveguide itself or the like, resulting in the deterioration in polarization-extinction ratio of an output signal. Therefore, in order to improve the quality of modulation signal, it is desirable to integrate a waveguide type polarizer of small size and low cost, which is capable of efficiently eliminating the unnecessary polarization, to the LN optical modulator.
As the waveguide type polarizer for greatly attenuating one of the polarization modes (TE/TM) propagated through the waveguide, there are known the ones by the following methods.
(1) By a Method of Disposing a Metal Film on the Waveguide (Via a Buffer Layer or the Like)
A waveguide type polarizer in which the leakage of the guided mode to a cladding (buffer layer) is utilized, to thereby utilize an effect in that a TM mode excites free electrons in the metal, to be largely attenuated in comparison with a TE mode (refer to the literature 1 “Optical Integrated Circuit (revised and enlarged edition)”, pp. 281-283, by Hiroshi NISHIHARA et al., Ohmsha, August 1993).
(2) By a Method of Utilizing a Directional Coupler
A waveguide type polarizer in which a propagation constant difference between the TE mode and the TM mode is utilized, to thereby utilize an optical power transition distance difference between two waveguides.
(3) By a Method of Using a Birefringent Material for a Cladding
A waveguide type polarizer in which a birefringent material is used for a cladding, so that the cladding has the refractive index which is higher than that of a core in one of polarized waves to achieve a radiation mode while being lower than that of the core in the other polarized wave to achieve a waveguide mode (refer to the literature b 1 and the literature 2: Japanese Unexamined Patent Publication No. 8-136753).
(4) By a Method of Using a Proton-Exchange Waveguide
A waveguide type polarizer in which only a TE mode or only a TM mode is propagated depending on a direction of a crystal substrate (refer to the literature 3: Japanese Unexamined Patent Publication No. 7-27935).
(5) By a Method of Using a Resonance Reflection Type Waveguide
A waveguide polarizer in which a low refractive index cladding, a high refractive index cladding and a core (having the refractive index same as that of the low refractive index cladding) are formed on a high refractive index substrate, to thereby confine a light by utilizing the resonance reflection of the high refractive index cladding, so that a high polarization-extinction ratio can be achieved by a film thickness control of the high refractive index cladding or the multi-layering thereof (refer to the literature 4: Japanese Unexamined Patent Publication No. 4-125602).
Each conventional waveguide type polarizer as described in the above can realize the high performance as a stand-alone polarizer. However, the following problems are caused, considering the integration or the unification of each polarizer with an optical waveguide device, such as the optical modulator or the like as described above.
Firstly, with regard to the waveguide type polarizer by the method of (1) or (2), as shown in an upper stage of FIG. 30 for example, since it is required to dispose a polarizer portion of a few millimeters to a few centimeters on an incident port side (or an emission port side) of the optical modulator or the like, the chip length is increased, resulting in the size enlargement of the entire optical modulator or in the loss degradation thereof. Further, if a modulator portion is shortened as shown in a lower stage of FIG. 30 in order to retain the chip length at the fixed length, the drive voltage is increased. In the case where the waveguide type polarizer by the method of (1) or (2) is used, to thereby realize the improvement of the polarization-extinction ratio of the optical waveguide device, such as the optical modulator or the like, there is a problem in that the various characteristics (for example, the chip length, an insertion loss, the drive voltage and the like) of the conventional optical waveguide device are degraded.
With regard to the waveguide type polarizer by the method of (3), if the birefringent material can be used for the cladding of the optical waveguide device, such as the optical modulator or the like, since the structure itself of the optical modulator or the like functions as the polarizer, it is possible to improve the polarization-extinction ratio without the degradation of the various characteristics as described above. However, considering the LN optical modulator for example, in an actual situation, there has not been known a material indicating the appropriate birefringence in the vicinity of the refractive index of LiNbO3. Even if such a material is obtained, it is necessary to match an orientation of the birefringent material to be used for the cladding with the crystal direction of the LiNbO3 substrate with high precision, and therefore, there is a problem in that such a waveguide is hard to be practically produced. Note, in the literature 2, there is proposed a technology for controlling the film producing process of the buffer layer to apply the stress on the buffer layer, thereby providing the anisotropy for the refractive index. However, there is a drawback in that the producing process becomes complicated, resulting in the cost increase.
With regard to the waveguide type polarizer by the method of (4), there is a problem in that the drive voltage (Vπ) is relatively high in the optical modulator to which the proton-exchange waveguide is applied.
With regard to the waveguide type polarizer by the method of (5), the structure of the waveguide is complicated, and a leakage amount (polarization-extinction ratio) of one of the polarization mode lights greatly depends on the film thickness of the cladding. Therefore, the film thickness control is significantly difficult, causing a problem in the producing performance.